CSS Help? Tools?

Hey all,

Finally got a plugin combo that is working to pull the most recent posts from each category into a slider of sorts. But I am having a terrible time getting the images to center in the viewing area.

How do you guys figure this stuff out? I've tried to use firebug a couple times without much success, is that the best tool to figure out what is wrong in css, etc stuff?

I can do all the CSS edits as far as the margins/padding etc for the images but I can't figure out which name they fall under to do the edits.

Thanks for any help,

Dani